1. Field of the Invention
Embodiments of the present invention relate to a top-emitting nitride-based light-emitting device, and a method of manufacturing the same, and more particularly, to a top-emitting nitride-based light-emitting device with improved ohmic characteristics and luminous efficiency and a method of manufacturing the same.
2. Description of the Related Art
Transparent conducting thin films are used for a wide variety of applications in optoelectronics, display, and energy industries. In the field of light-emitting devices, research is being actively conducted around the world to develop a transparent conducting ohmic electrode structure that allows efficient hole injection and light emission.
Transparent conducting oxides (TCOs) and transparent conducting nitrides (TCNs) are currently attracting a great deal of interest. Typical examples of TCO are indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), and indium tin oxide (ITO). The most commonly used TCN is a titanium nitride (TiN). However, using only TCO or TCN film as a p-type transparent ohmic electrode for a top-emitting gallium nitride (GaN)-based light-emitting device may cause many problems since the TCO or TCN film exhibits high sheet resistance, high light reflectivity, and low work function under conditions for film formation.
The first problem is that the above-enumerated transparent conducting thin films are not suitable for use in large-area, large-capacity, and high-brightness light-emitting devices because their high sheet resistance of approximately 100 Ω/cm2 will hinder current spreading along a lateral direction (parallel to interlayer between layers) and vertical hole injection during film formation using physical vapor deposition (PVD) such as sputtering, electron-beam (e-beam) or heat evaporation, or pulsed laser deposition (PLD).
The second problem is that a p-GaN surface being exposed to plasma ions when the transparent conducting film is deposited using plasma can be easily damaged, thus degrading electrical characteristics.
The third problem is that the luminous efficiency of the transparent conducting film decreases due to its high reflectivity with respect to light emitted by a GaN-based light-emitting device.
The last problem is that when a TCO is used as an electrode in direct ohmic contact with a GaN-based compound semiconductor, insulating gallium oxide (Ga2O3) and magnesium oxide (MgO) are formed on the GaN surface during deposition of the thin film on the GaN due to the strong oxidizing power of Ga and Mg (a p-type dopant), thereby making it difficult to achieve a high quality ohmic contact electrode.
Meanwhile, light-emitting devices are classified into top-emitting light-emitting diodes (TLEDs) and flip-chip LEDs (FCLEDs). In commonly used TLEDs, light exits through an ohmic contact layer in contact with a p-cladding layer. However, due to the high sheet resistance of a p-cladding layer with a low hole concentration, a current spreading layer with a good current spreading capability is essentially required in order to achieve a high brightness TLED. That is, a current spreading layer with low sheet resistance and high light transmittance must be used as an ohmic contact layer to provide excellent hole injection, current spreading, and light emission.
TLEDs typically use a structure in which a nickel (Ni)/gold (Au) ohmic contact layer is formed on a p-cladding layer. The Ni/Au ohmic contact layer annealed in an oxygen (O2) ambient has been known to have excellent specific contact resistivity of approximately 10−4 to 10−3 Ωcm2 and semi-transparency. When the conventional ohmic contact layer is annealed at temperature of 500 to 600° C. in an O2 ambient, a nickel oxide (NiO) that is a p-type semiconductor oxide is formed on the island-like Au layer and between the Au layer and a p-GaN, thereby reducing a Schottky barrier height (SBH) at the interface between the p-GaN cladding layer and the Ni ohmic contact layer. Thus, holes that are majority carriers can be easily injected into the surface of the p-cladding layer.
Furthermore, annealing of the Ni/Au ohmic contact layer on the p-cladding layer results in disassociation of a Mg—H complex in GaN, which reactivates Mg dopants by increasing the concentration on the surface of GaN. As a result of reactivation, effective carrier concentration increases above 10 weight percent on the surface of the p-cladding layer. This causes tunneling conductance between the p-cladding layer and the ohmic contact layer containing NiO, thus achieving an ohmic contact with good ohmic conductance and low specific contact resistivity.
However, due to their low luminous efficiency, TLEDs using a semi-transparent Ni/Au ohmic contact layer suffer the limitation of not being able to realize the next generation light-emitting devices with large capacity and high brightness.
In a FCLED design, light is emitted through a sapphire substrate using a reflective layer in order to increase the amount of heat emitted during its operation as well as luminous efficiency. However, the FCLED also suffers from problems such as high resistance due to poor adhesion and oxidation of the reflective layer.
Thus, as a solution to overcome the limitations of TELDs and FCLEDs, the use of a TCO completely excluding Au, such as indium tin oxide (ITO), with superior light transmittance over a semi-transparent Ni (NiO)/Au used as a conventional p-ohmic contact layer, has been proposed in various literatures [IEEE PTL, Y. C. Lin, etc. Vol. 14, 1668 and IEEE PTL, Shyi-Ming Pan, etc. Vol. 15, 646]. The development of a TLED with an ITO ohmic contact layer exhibiting improved output power over a conventional one employing a Ni/Au ohmic contact has been recently discussed in a paper [Semicond. Sci. Technol., C S Chang, etc. 18 (2003), L21]. Furthermore, U.S. Pat. No. 6,297,947 proposes a method of fabricating a LED with improved light transmittance and electrical characteristics by combining thin oxidized Ni/Au or Ni/silver (Ag) with ITO. However, the proposed method is not suitable for use in high volume applications because of high specific contact resistivity and addition of the step of oxidizing the Ni/Au (or Ag).
As described above, there are the following fundamental problems that make it difficult to develop a high quality ohmic contact electrode.
First, a p-GaN has high sheet resistance above 104 Ω/cm2 due to its low hole concentration.
Second, a SBH is increased at the interface between a p-GaN and an electrode due to the absence of a transparent electrode material having a work function higher than the p-GaN, thereby hindering vertical hole injection.
Third, like most materials having an inverse relationship between electrical and optical characteristics, transparent electrodes with high light transmittance have high sheet resistance, thereby significantly reducing lateral current spreading.
Fourth, insulating Ga2O3 and MgO are formed on a GaN surface when a transparent conducting layer is deposited directly on the p-GaN, thereby degrading electrical characteristics of a light-emitting device.
Fifth, the surface of a p-GaN can be easily damaged by plasma during sputtering that allows for formation of a transparent conducting layer with a low sheet resistance.
Due to the above-mentioned problems, the amount of heat generated between the p-GaN and the ohmic contact layer increases, thus decreasing the life span and reliability of a light-emitting device.